1. Field of the Invention
The present invention relates to homogeneous silicon nitride sintered bodies which are useful for bearing members, wear resistant members, frictionally moving members, etc., and manufacturing processes thereof.
2. Description of Prior Art
Heretofore, silicon nitride sintered bodies have been manufactured by processes as follows:
On the outset, a silicon nitride starting material is mixed with a sintering aid and after pulverization passed through usually a 44 .mu.m meshed sieve to remove foreign matter, such as fragments of pebbles used during the pulverization, or the like. Then, the sieved starting material mixture is granulated and aged or added with water to control its water content. After further sieving, the mixture is molded by means of mold press or cold isostatic press and then fired at a predetermined temperature to provide sintered bodies.
However, in such conventional processes as mentioned above for manufacturing silicon nitride sintered bodies, neither removal of coarse grains remaining after the pulverization and foreign matter contained in the starting material nor uniformizing of water content in the granulated powder has been positively conducted, so that the following problems that require solution have been involved. Namely, in the conventional processes, there have been cases where the coarse grains and the foreign matter contained in the starting material are incorporated into molded bodies or the water content of the granulated powder is varied. In consequence, the incorporation of the coarse grains and the foreign matter contained in the starting material or the water content variation has caused uneven grain disintegration, resulting in drawbacks, such as the formation of pores which remain in the molded bodies after sintering, or growth of a crystalline phase occupying more than a certain proportion in grain boundary, whereby homogeneous silicon nitride sintered bodies have not been obtainable.
In the case of application particularly to bearing members, wear resistant members or frictionally moving members, the proportion of the crystalline phase in the grain boundary largely affects those members' lives, so that it has been required to manufacture sintered bodies that have a crystalline phase not exceeding a certain proportion in the grain boundary in order to provide the resulting sintered bodies with a life longer than conventional ones. In the case of use for, inter alia, bearing members, it has been known to be important to grasp a rolling fatigue life of its material. Accordingly, development of a material that is homogeneous and of high tenacity has been required for improving the rolling fatigue life.